Binary black hole spin measurements from gravitational wave observations can reveal the binary’s evolutionary history. In particular, the spin orientations of the component black holes within the orbital plane, ${\varphi }_1$ and ${\varphi }_2$, can be used to identify binaries caught in the so-called spin-orbit resonances. In a companion paper, we demonstrate that ${\varphi }_1$ and ${\varphi }_2$ are best measured near the merger of the two black holes. In this work, we use these spin measurements to provide the first constraints on the full six-dimensional spin distribution of merging binary black holes. In particular, we find that there is a preference for $\mathrm{\Delta }\varphi ={\varphi }_1-{\varphi }_2\sim \pm \pi$ in the population, which can be a signature of spin-orbit resonances. We also find a preference for ${\varphi }_1\sim -\pi /4$ with respect to the line of separation near merger, which has not been predicted for any astrophysical formation channel. However, the strength of these preferences depends on our prior choices, and we are unable to constrain the widths of the ${\varphi }_1$ and $\mathrm{\Delta }\varphi$ distributions. Therefore, more observations are necessary to confirm the features we find. Finally, we derive constraints on the distribution of recoil kicks in the population and use this to estimate the fraction of merger remnants retained by globular and nuclear star clusters. We make our spin and kick population constraints publicly available.

• Received 23 July 2021
• Revised 29 September 2021
• Accepted 17 December 2021

DOI:https://doi.org/10.1103/PhysRevLett.128.031101

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

Published by the American Physical Society